Viscoelastic surfactant fluids are normally made by mixing in appropriate amounts suitable surfactants such as anionic, cationic, nonionic and zwitterionic surfactants in an aqueous medium. The rheology of viscoelastic surfactant fluids, in particular the increase in viscosity of the solution, is attributed to the three dimensional structure formed by the components in the fluids. When the surfactant concentration significantly exceeds a critical level, and eventually subject to the presence of an electrolyte, the surfactant molecules aggregate and form structures such as micelles that can interact to form a network exhibiting viscoelastic behavior. In the remaining part of this description, the term “micelle” will be used as a generic term for organized interacting species.
Viscoelastic surfactant solutions are usually formed by the addition of certain reagents to concentrated solutions of surfactants, frequently consisting of long-chain quaternary ammonium salts such as cetyltrimethylammonium bromide (CTAB). Common reagents that generate viscoelasticity in the surfactant solutions are salts such as ammonium chloride, potassium chloride, sodium salicylate and sodium isocyanate and non-ionic organic molecules such as chloroform. The electrolyte content of surfactant solutions is also an important control on their viscoelastic behavior.
There has been considerable interest in using such viscoelastic surfactants in wellbore-service applications. Reference is made for example to U.S. Pat. Nos. 4,695,389; 4,725,372; 5,551,516, 5,964,295, and 5,979,557.
The rheological properties of aqueous mixtures of surfactants are determined by their tendency to seclude their hydrophobic part, and expose their hydrophilic part, toward the solvent. This behavior typically results in the formation of three-dimensional network structure, called micelles. Depending in particular upon the structure of these micelles, the fluid viscosity is more or less increased, and the fluid may exhibit both viscous and elastic behavior.
The common approach to develop new viscoelastic-surfactant systems is to screen a large number of surfactants—and surfactant mixtures—until one meets specific performance specifications. This approach is obviously time-consuming. Moreover, wellbore services fluids tend to be used under a large variety of conditions, notably temperature, salinity and shear stress. Unfortunately, viscoelastic-surfactants based-fluids are typically very sensitive to variations of the above-mentioned parameters. Therefore the “screening” approach tends to result in numerous systems that are tailored for specific conditions. This presents logistical issues and requires extensive training of field personnel.
Consequently, it would be desirable to have one system whose properties could be adjusted to meet a variety of specifications. For example, consider the possibility of using a particular viscoelastic surfactant system throughout a broad temperature range. It is known that the micelles responsible for the theological properties of viscoelastic surfactant-based fluids are normally stable within a narrow temperature range. Surfactants with longer carbon-atom hydrophobic chains (more than 18 carbon atoms) offer fluid stability at higher temperatures. However, increasing the chain length is also detrimental to the surfactant's hydrophilic properties; therefore, complete dissolution of the surfactant requires considerably more time than that of shorter chain counterparts. There is therefore a need for means to “boost” the viscosity of shorter-chain systems at higher temperatures. It should be further emphasized that some relatively inexpensive viscoelastic surfactants may provide an increase of viscosity that is less than it would be desirable for some applications. Providing means to boost the viscosity would be a way of allowing the use of “less than perfect” product—or to limit the quantity of surfactant to be added to the systems and therefore decreasing the total cost of the system.
Another property of viscoelastic surfactant-based systems is their shear sensitivity. For instance, in the oil industry, it is often favorable to provide fluids that exhibit high viscosity at little or no shear and low viscosity at high shear. Such fluids are easy to pump but will be highly viscous after placement in the well. Though the shear-sensitivity is an intrinsic property of most viscoelastic systems, an independent aspect is the degree of viscosity-recovery or re-healing once the fluid is no more subject to high shear. Controlling the degree of reassembling (re-healing) is necessary to maximize performance of the surfactant system for different applications. For example, in hydraulic fracturing it is critical for the fluid to regain viscosity as quickly as possible after exiting the high-shear region in the tubulars and entering the low-shear enviroment in the hydraulic fracture. On the other hand, it is beneficial in coiled tubing cleanouts to impart a slight delay in regaining full viscosity in order to more efficiently “jet” the solids from the bottom of the wellbore into the annulus. Once in the annulus the regained viscosity will ensure that the solids are effectively transported to the surface. Improving the viscosity-recovery and minimizing the time required for such recovery is therefore desirable.
Finally, it is well known that the introduction of certain components to a viscoelastic surfactant-based system can cause a dramatic decrease in the fluid viscosity, called “breaking”. Breaking can also occur by varying the amount of water or electrolyte or other components that may already be present in the fluid. For example, in oilfield applications, the viscosity of viscoelastic surfactant fluids is reduced or lost upon exposure to formation fluids (e.g., crude oil, condensate and/or water). The viscosity reduction effectuates cleanup of the reservoir, fracture, or other treated area.
However, in some circumstances, it would be suitable to have a better control of that breaking, for instance, when breaking of the fluid is desired at a particular time or condition, when it is desired to accelerate viscosity reduction or when the natural influx of reservoir fluids (for example, in dry gas reservoirs) does not break or breaks incompletely the viscoelastic surfactant fluid.
This disclosure describes compositions and methods employed to modify the rheology of aqueous solutions comprising a thickening amount of a viscoelastic surfactant.
UK Patent GB2332223, “Viscoelastic surfactant based gelling composition for wellbore service fluids” by Hughes, Jones and Tustin describes methods to delay and control the build-up of viscosity and gelation of viscoelastic surfactant based gelling compositions. These methods are used to facilitate placement of the delayed (“pre-gel”) fluid into a porous medium and then to trigger formation of the viscoelastic gel in-situ.
Rose et. al. describe in U.S. Pat. No. 4,735,731 several methods to reversibly break the viscosity of viscoelastic-surfactant based solutions through an intervention at surface. These methods include heating/cooling the fluid, adjusting the pH or contacting the fluid with an effective amount of a miscible or immiscible hydrocarbon and then, subjecting the fluid to conditions such that the viscosity of the fluid is substantially restored. The reversible treatment of Rose is useful for drilling fluids so that the fluid pumped into the well is viscous enough to carry cuttings to the surface but able to be broken at surface for solids removal. The breaking methods discussed in Rose are not used to break a viscoelastic solution down a well and further appear to have an immediate impact on the viscosity of the fluid.
U.S. patent application Ser. No. 09/826,127 filed Apr. 4, 2001 and published under Ser. No. 20020004464 discloses different types of breaking agents and different means to achieve a delayed release of the breaking agents downhole so that the rheological properties of the aqueous fluids are not altered at surface or during the injection phase. U.S. application Ser. No. 10/194,522 filed Jul. 12, 2002 further discloses that some polymers, in particular some polyelectrolytes, can be used as breaking agents.
However, it was further found that the same types of polymers could also have completely different effects on the rheology of aqueous solutions comprising thickening amount of viscoelastic surfactants. Therefore, there exists a need for methods for breaking/enhancing/healing viscoelastic surfactant fluids after subterranean oil- or gas-well treatments, at predetermined times or conditions.